Field of the Invention
The invention lies in the automotive arts. Specifically, the invention relates to a control device for a fuel injection system, in particular for driving the power output stage of a fuel pump or of a fuel injection valve of an internal combustion engine.
Switching signals for an external electronic power system, by means of which a power output stage and thus, for example, a (diesel) fuel pump or fuel injection valves are activated, are output at the output side of a control device. In order to control the quantity of fuel precisely, the time characteristics of the switching signals must be precise and stable, even if the input impedance of the electronic power system can fluctuate within wide ranges for fabrication reasons. The trailing switching signal edge is functionally of particular importance here.
The following requirements are made of such a control device: signal level Low &lt;0.9V; High &gt;3.3V;
Voltage switching edge 3 .mu.s with max. tolerance of .+-.1.5 .mu.s; PA1 Temperature range: -40.degree. C. . . . +125.degree. C.; PA1 Input impedance of the electronic power system: PA1 a supply voltage source and series circuit connected between the poles of the supply voltage source, the series circuit including a bipolar pnp-type transistor highside switch, a first resistor, and a first capacitor; PA1 an output connected between the first resistor and the first capacitor; PA1 a lowside switch connected in series with the highside switch between the poles of the supply voltage source; PA1 a second capacitor connected between the highside switch and the lowside switch; PA1 a second series circuit connected in parallel with the second capacitor and including the first resistor, a second resistor, and a third capacitor; and PA1 a further resistor connected between the supply voltage terminal and the lowside switch.
10 k.OMEGA.&lt;Rin &lt;1M.OMEGA., 1nF &lt;Cin &lt;3nF (a further 1nF capacitor --C3 -- is also present in the engine control unit for the sake of EMV suppression); PA2 Protection of the output against short-circuiting to ground; PA2 Minimum leakage currents when the ground potential at the control unit is lost ("loss of ground").
In addition to the short-circuit withstand capability which is necessary in the field of motor vehicles, in the case of "loss of ground" the electronic power system must not switch on under any circumstances; it would result in a static high level at the input of the electronic power system and thus in an uncontrolled supply of fuel, which could lead to damage to the engine and to persons.
Previous circuits have used a pnp-type transistor as highside switch with a series resistor between the collector and output and in addition a MOS-FET as lowside switch between the output and ground in order to discharge quickly the EMC capacitance and the input capacitance of the electronic power system connected downstream. This serves to generate a trailing switching signal edge with the required time characteristics.
Since the drain terminal of the lowside switch is connected directly to the output, overcurrent disconnection must be provided as a protection against short-circuiting to the battery. This is part of a complex switching IC which is used.
The required disconnection in the event of "loss of ground" is brought about by inserting a diode in series with the lowside switch. However, as a result of the additional diode forward voltage, the required low level (&lt;0.9V) can no longer be maintained at low temperatures.